(NewsTarget) According to the National Institute of Neurological Disorders and Stroke, most researchers believe exposure to some kind of toxin or toxins in the environment triggers the development of Parkinson's disease (PD) -- the degenerative disorder of the central nervous system that impairs motor skills (including walking), speech and other functions. Pesticides have long been on the list of possible suspects as a PD-causing toxin. But a new study just published in the American Journal of Epidemiology by University of California at Los Angeles (UCLA) scientists appears to be the "smoking gun" that places pesticides at the top of that list. They found that exposure to a combination of two widely used pesticides increased the risk of Parkinson's disease by an incredible 75 percent.

In previous animal studies and cell cultures, researchers have shown pesticides spark a neurodegenerative process that leads to Parkinson's disease. The UCLA scientists, however, are the first to provide evidence for a similar process in humans.

They came up with their alarming results by analyzing an epidemiological study of Central Valley, California, residents. The region is one of the nation's top food-growing regions and crops like potatoes, dry beans and tomatoes have long been routinely sprayed with fungicides, herbicides and pesticides.

For their study, the UCLA researchers enrolled 368 longtime residents of Central Valley who had been diagnosed with Parkinson's and 341 other PD-free residents as a control group. Their analysis found that people who lived within 500 meters of fields sprayed with two pesticides, maneb and paraquat, between 1974 and 1999 had an extraordinary 75 percent increased risk for Parkinson's. What's more, people who were diagnosed with PD by the age of 60 or younger were found to have been at much higher risk because they had been exposed to maneb, paraquat or both in combination between 1974 and 1989, during the time they were children, teens or young adults. In these people, early pesticide exposure had increased their risk for the disease by four to six times.

"The results confirmed two previous observations from animal studies. One, that exposure to multiple chemicals may increase the effect of each chemical. That's important, since humans are often exposed to more than one pesticide in the environment. And second, that the timing of exposure is also important," UCLA scientist Beate Ritz, professor of epidemiology at the UCLA School of Public Health, said in a statement to media about her research team's findings.

She added that this is the first epidemiological study to provide strong evidence that maneb and paraquat work together to become highly neurotoxic in humans and greatly increase the risk of PD. What makes this particularly concerning is that the UCLA data "suggests that the critical window of exposure to toxicants may have occurred years before the onset of motor symptoms when a diagnosis of Parkinson's is made," Dr. Ritz said.